There are multiple orders of jellyfish, and each one branches off into smaller genera and species. Jellyfish are invertebrates, and like all cnidarians, are radial symmetrical, meaning their body is symmetrical around a central axis. Although jellyfish vary physiologically from species to species, they all have a top, or bell, and thin tentacles that grow from the bottom edge of the bell. Barbs on their tentacles that they use to sting and immobilize their prey. (2)
Relationship to humans
Jellyfish sting other animals, including humans. Also, humans can eat jellyfish.
Habitat and niche
Almost all species of jellyfish live in saltwater such as oceans. However, based on the species, jellyfish live at different depths. Some live in shallow waters, for example: harbors, bays, and estuaries. These shallow water jellyfish form a symbiotic relationship with uni-cellular algae. The algae, zooxanthelle, sits on the underside of the jellyfish's belly and provides the jellyfish with nourishment through the process of photosynthesis. Therefore, these jellyfish stay close to the surface to ensure sunlight for their photosynthetic algae. Other types of jellyfish live in deep-sea habitats lower than 1000 meters, benefiting from its darkness and low temperature. One species of jellyfish, Craspedacusta sowerbyi, or freshwater jellyfish, resides in freshwater bodies such as lakes, algae-infested ponds, rivers, and reservoirs (1).
Predator avoidance
Uses a toxin stored in nematocysts that can paralyze predators (13). Jellyfish are equipped with a specialized venom apparatus (cnidoblast) for defense. The capsule (nematocyst) inside the cnidoblast contains a trigger and a stinging structure, which consists of a hollow coiled thread lined with barbs. Each tentacle can be embedded with thousands of nematocysts, which are activated when the tentacles make contact with an object. Pressure in the nematocyst forces the stinging thread to rapidly uncoil, injecting paralyzing toxins into predators or prey. (4)
This method of capturing prey also applies to the method of avoiding predators for jellyfish, as described above. (12)
Nutrient acquisition
Jellyfish are carnivores that use a toxin in their nematocysts, which are harpoon-like structures on the jellyfish, which can deliver a paralyzing shock. This allows it to capture large prey.
Jellyfish do not have a complete gut; their mouth is attached to a gastrovascular cavity that functions in digestion along with other roles and has a single opening that serves as both a mouth and anus (13).
Reproduction and life cycle
The jellyfish life cycle has two distinct stages: a sessile stage and a motile stage. In the sessile polyp stage, it is just a cylindrical stalk attached to the ocean floor that produces medusa through asexual budding. Once it enters the motile medusa phase, the jellyfish is a free-swimming organism that is shaped like a bell. The medusa is sexual and produces eggs or sperm by meiosis and releases the gametes into the water. A fertilized egg develops into a larva called a planula. This planula eventually settles to the bottom of the ocean and becomes a polyp (13).
Growth and development
After fertilization, early embryonic development occurs either in the female or in brood pouches. Small swimming larvae (planulae) leave the mouth or pouch to enter the water where they then attach themselves to something firm on the ocean floor and gradually develop into polyps. These polyps use tentacles to feed on microscopic organisms in the water and can multiply by budding to produce more polyps. When the conditions are right, developed polyps form constrictions in their bodies that begin a larval phase that resembles a stack of saucers. Each "saucer" ring separates from the pack and becomes a free swimming organism on its own. In a few weeks, this organism will grow into an adult jellyfish, called a medusa. Jellyfish normally live for a few months, but the polyp stage may be perennial (14).
Integument
Jellyfish has two layers of skin: the epidermis, or the outer layer, and the gastrodermis, or the gut lining. A layer of thick gelatinous substance called mesoglea fills the portion between these two skin layers. Jellyfish do not have a hard outer shell. In fact, jellyfish get their name from the soft, jelly like texture they possess. Their soft bodies allow them to swim actively and very delicately through waters. (7) Movement
Jellyfish have epithelial cells with muscle fibers that contract, allowing it to move. They also contain simple nerve nets. The ability to contract muscle fibers gives the jellyfish control over its vertical movement, but it is heavily reliant on currents to move from side to side. (9).
Sensing the environment
Jellyfish have an organ similar to an eye called the rhopalium, which are dark spots located on the edge of the jellyfish, the top of the jellyfish, and vary in number depending on the type of jellyfish.
Jellyfish have a network of nerve clusters known as "nerve nets" that, although they don't make up a true central nervous system, allow the jellyfish to sense environmental stimuli such as the touch of another animal and respond with protective or escape movements. (5)
Gas exchange
The exchange of gases occurs on the surface of jellyfish by diffusion: the concentration of oxygen inside the organism is lower than the concentration of the air or water. Similarly, the carbon dioxide exchange also occurs by diffusion since CO2 levels are higher inside the organisms than in the environment. The oxygen enters the jellyfish through the epidermis. Once there, it diffuses into the mesoglea where the oxygen is absorbed and transported to the metabolic region of the jellyfish. The mesoglea also has the ability to store oxygen when the jellyfish enters an environment of low O2 concentration (10).
Waste removal
The gastrovascular cavity has a single opening, so that is where nutrients enter and wastes exit (13).
Environmental physiology (temperature, water and salt regulation)
Jellyfish do not have special structures to maintain homeostasis as a jellyfish's cells exchange nutrients and wastes directly with the environment. Jellyfish are largely dependent on their environment to maintain proper temperature, pH, chemical, and salt balance because they can not regulate these substances on their own (15).
Internal circulation
The internal circulation of a jellyfish is somewhat limited. Jellyfish have an opening where the food is able to enter and the mouth is the opening to a cavity. This cavity contains cilia which help to circulate the water and dissolved substances throughout the body of the jellyfish.
Jellyfish do not have a circulatory system. They absorb oxygen through their thin bodies (8).
Chemical control (i.e. endocrine system)
Not much is known about the jellyfish endocrine system as it does not have a heart and is mostly made up of water. It contains very little proteins and minerals. Their sensory organs, rhopalia, may have something to do with their endocrine system, but further studies must be done to determine whether or not this is correct (16).
Jellyfish (cnidaria)
By Kristine Moran
Classification/Diagnostic characteristics
Domain: Eukarya
There are multiple orders of jellyfish, and each one branches off into smaller genera and species. Jellyfish are invertebrates, and like all cnidarians, are radial symmetrical, meaning their body is symmetrical around a central axis. Although jellyfish vary physiologically from species to species, they all have a top, or bell, and thin tentacles that grow from the bottom edge of the bell. Barbs on their tentacles that they use to sting and immobilize their prey. (2)
Relationship to humans
Jellyfish sting other animals, including humans. Also, humans can eat jellyfish.Habitat and niche
Almost all species of jellyfish live in saltwater such as oceans. However, based on the species, jellyfish live at different depths. Some live in shallow waters, for example: harbors, bays, and estuaries. These shallow water jellyfish form a symbiotic relationship with uni-cellular algae. The algae, zooxanthelle, sits on the underside of the jellyfish's belly and provides the jellyfish with nourishment through the process of photosynthesis. Therefore, these jellyfish stay close to the surface to ensure sunlight for their photosynthetic algae. Other types of jellyfish live in deep-sea habitats lower than 1000 meters, benefiting from its darkness and low temperature. One species of jellyfish, Craspedacusta sowerbyi, or freshwater jellyfish, resides in freshwater bodies such as lakes, algae-infested ponds, rivers, and reservoirs (1).Predator avoidance
Uses a toxin stored in nematocysts that can paralyze predators (13). Jellyfish are equipped with a specialized venom apparatus (cnidoblast) for defense. The capsule (nematocyst) inside the cnidoblast contains a trigger and a stinging structure, which consists of a hollow coiled thread lined with barbs. Each tentacle can be embedded with thousands of nematocysts, which are activated when the tentacles make contact with an object. Pressure in the nematocyst forces the stinging thread to rapidly uncoil, injecting paralyzing toxins into predators or prey. (4)This method of capturing prey also applies to the method of avoiding predators for jellyfish, as described above. (12)
Nutrient acquisition
Jellyfish are carnivores that use a toxin in their nematocysts, which are harpoon-like structures on the jellyfish, which can deliver a paralyzing shock. This allows it to capture large prey.Jellyfish do not have a complete gut; their mouth is attached to a gastrovascular cavity that functions in digestion along with other roles and has a single opening that serves as both a mouth and anus (13).
Reproduction and life cycle
The jellyfish life cycle has two distinct stages: a sessile stage and a motile stage. In the sessile polyp stage, it is just a cylindrical stalk attached to the ocean floor that produces medusa through asexual budding. Once it enters the motile medusa phase, the jellyfish is a free-swimming organism that is shaped like a bell. The medusa is sexual and produces eggs or sperm by meiosis and releases the gametes into the water. A fertilized egg develops into a larva called a planula. This planula eventually settles to the bottom of the ocean and becomes a polyp (13).Growth and development
After fertilization, early embryonic development occurs either in the female or in brood pouches. Small swimming larvae (planulae) leave the mouth or pouch to enter the water where they then attach themselves to something firm on the ocean floor and gradually develop into polyps. These polyps use tentacles to feed on microscopic organisms in the water and can multiply by budding to produce more polyps. When the conditions are right, developed polyps form constrictions in their bodies that begin a larval phase that resembles a stack of saucers. Each "saucer" ring separates from the pack and becomes a free swimming organism on its own. In a few weeks, this organism will grow into an adult jellyfish, called a medusa. Jellyfish normally live for a few months, but the polyp stage may be perennial (14).Integument
Jellyfish has two layers of skin: the epidermis, or the outer layer, and the gastrodermis, or the gut lining. A layer of thick gelatinous substance called mesoglea fills the portion between these two skin layers. Jellyfish do not have a hard outer shell. In fact, jellyfish get their name from the soft, jelly like texture they possess. Their soft bodies allow them to swim actively and very delicately through waters. (7)Movement
Jellyfish have epithelial cells with muscle fibers that contract, allowing it to move. They also contain simple nerve nets. The ability to contract muscle fibers gives the jellyfish control over its vertical movement, but it is heavily reliant on currents to move from side to side. (9).
Sensing the environment
Jellyfish have an organ similar to an eye called the rhopalium, which are dark spots located on the edge of the jellyfish, the top of the jellyfish, and vary in number depending on the type of jellyfish.Jellyfish have a network of nerve clusters known as "nerve nets" that, although they don't make up a true central nervous system, allow the jellyfish to sense environmental stimuli such as the touch of another animal and respond with protective or escape movements. (5)
Gas exchange
The exchange of gases occurs on the surface of jellyfish by diffusion: the concentration of oxygen inside the organism is lower than the concentration of the air or water. Similarly, the carbon dioxide exchange also occurs by diffusion since CO2 levels are higher inside the organisms than in the environment. The oxygen enters the jellyfish through the epidermis. Once there, it diffuses into the mesoglea where the oxygen is absorbed and transported to the metabolic region of the jellyfish. The mesoglea also has the ability to store oxygen when the jellyfish enters an environment of low O2 concentration (10).Waste removal
The gastrovascular cavity has a single opening, so that is where nutrients enter and wastes exit (13).Environmental physiology (temperature, water and salt regulation)
Jellyfish do not have special structures to maintain homeostasis as a jellyfish's cells exchange nutrients and wastes directly with the environment. Jellyfish are largely dependent on their environment to maintain proper temperature, pH, chemical, and salt balance because they can not regulate these substances on their own (15).Internal circulation
The internal circulation of a jellyfish is somewhat limited. Jellyfish have an opening where the food is able to enter and the mouth is the opening to a cavity. This cavity contains cilia which help to circulate the water and dissolved substances throughout the body of the jellyfish.Jellyfish do not have a circulatory system. They absorb oxygen through their thin bodies (8).
Chemical control (i.e. endocrine system)
Not much is known about the jellyfish endocrine system as it does not have a heart and is mostly made up of water. It contains very little proteins and minerals. Their sensory organs, rhopalia, may have something to do with their endocrine system, but further studies must be done to determine whether or not this is correct (16).References:
1. http://www.jellyfishfacts.net/jellyfish-habitat.html
2. http://animals.about.com/od/cnidarians/p/jellyfish.htm
3. http://www.dailyherald.com/article/20130320/news/703209834/
4. http://www.dnr.sc.gov/marine/pub/seascience/jellyfi.html
5. http://jeb.biologists.org/content/214/8/1215.long
6. http://bioweb.uwlax.edu/bio203/s2007/shull_step/classification.htm
7. http://www.jellyfishfacts.net/jellyfish-body.html
8. http://thecircleofblood.weebly.com/cnidaria.html
9. http://www.dnr.sc.gov/marine/pub/seascience/jellyfi.html
10. http://rpi-cloudreassembly.transvercity.net/2012/11/05/oxygenation-of-jellyfish/
11. http://www.ucmp.berkeley.edu/cnidaria/scyphozoamm.html
12. http://amper.ped.muni.cz/~jonas/biologie/Biologie/48%20obr/33_06HydraCnidocyte_L.jpg
13. Biology of Life Textbook
14. http://www.dnr.sc.gov/marine/pub/seascience/jellyfi.html
15. http://www.ehow.com/info_10031510_jellyfish-maintain-homeostasis.html
16. http://evolutionofsystems.weebly.com/cnidaria.html
Picture 1: http://upload.wikimedia.org/wikipedia/commons/b/bf/Aurelia_aurita_1.jpg
Picture 2: http://www.ogita.jp/wp-content/uploads/Aquarium20.jpg
Rhopalium Picture: http://www1.bio.ku.dk/forskning/forskningsnyt/daugbjerg/ or http://www1.bio.ku.dk/forskning/forskningsnyt/daugbjerg/goples.jpg/